Xanthine oxidase decreases production of gut wall nitric oxide.

Multiorgan failure is often the lethal outcome of intratracheal aspiration of acidic gastric juice. The pathogenesis of multiorgan failure may involve a systemic imbalance between pro-inflammatory and anti-inflammatory factors. In an anesthetized rat model, intratracheal instillation of HCl elicited intestinal inflammation which was exaggerated by xanthine oxidase (XO) and attenuated by nitric oxide (NO). We hypothesized that XO may mediate injury in part by suppression of NO formation. Therefore, we measured intestinal tissue concentrations of the stable NO oxidative metabolites (NO2- and NO3-) following intratracheal (IT) instillation of NaCl or HCl alone or in combination with interventions aimed at increasing or decreasing XO activity. Compared with IT NaCl (control treatment) jejunal tissue NO2- and NO2- + NO3- concentrations were increased by allopurinol pretreatment, which inhibits XO, and were decreased by systemically administered XO, as well as by IT HCl. The decreased NO2- and NO2- + NO3- concentrations found following IT HCl were completely reversed by either allopurinol or by systemically administered L-arginine (the precursor of NO). We conclude that manipulation of the pro-inflammatory XO system has a reciprocal effect on the intestinal anti-inflammatory NO system in either the undamaged or the endobronchially acidified lung model.

G protein-coupled receptor signaling: implications for the digestive system.

Extracellular signaling molecules regulate intracellular events by way of complex transduction assemblies composed of several proteins: receptor, G protein, effector, inactivating enzyme. Much is known about the structure and function of these transducer proteins. A signaling molecule initiates transduction by binding to the receptor which then prompts the G protein to undergo a reaction cycle. This cycle involves guanine nucleotide binding and hydrolysis, G protein subunit dissociation, and interactions with an effector (e.g. adenylyl cyclase, phospholipase C), as well as with inactivating molecules. The result is altered generation of intracellular second messengers, protein transcription, or another profound cellular response. This signal transduction system also contains multiple mechanisms for turning off the signal such as phosphorylating, internalizing, or downregulating receptors, uncoupling the receptor-G protein complex, or cell-surface peptidases, and precipitating conformational changes in transducer elements. These aspects of signal transduction are examined in two well studied systems, namely the beta 2-adrenergic and the substance P transducers. Both complexes are important physiological neuroregulators in the gut and elsewhere. Pathophysiological mechanisms involving aberrent signal transduction have been implicated in various diseases including major common illnesses such as heart failure and gastrointestinal disorders such as cholera, other infectious diarrheas, and colitis.

Nitric oxide attenuates and xanthine oxidase exaggerates lung damage-induced gut injury.

Aspirated gastric contents can evoke multiorgan failure. We hypothesized that secondary intestinal epithelial dysfunction after lung damage would be mediated by xanthine oxidase (XO) and antagonized by endogenous gut nitric oxide (NO). Isosmotic saline or HCl solutions were instilled intratracheally in anesthetized rats, and intestinal injury was assessed 190 min later by measuring the blood-to-lumen clearance of 51Cr-labeled EDTA (51Cr-EDTA clearance) and gut wall neutrophil population density. Intratracheal HCl increased 51Cr-EDTA clearance, and this transepithelial leak was attenuated by either systemic L-arginine or intraluminal NO and by chronic dietary pretreatment with allopurinol or sodium tungstate. Conversely, lung damage-induced gut leak was exaggerated by NO synthase inhibition or intravenous XO administration. Intratracheal HCl also increased intestinal wall neutrophil density and myeloperoxide activity. We conclude that two enzymatic systems involved in remote gut barrier dysfunction after endobronchial acidification are XO as mediator and NO synthase as antagonist.

Nitric oxide decreases lung injury after intestinal ischemia.

After injury to a primary organ, mediators are released into the circulation and may initiate inflammation of remote organs. We hypothesized that the local production of nitric oxide (NO) may act to limit the spread of inflammation to secondarily targeted organs. In anesthetized rats, 30 min of intestinal ischemia followed by 2 h of reperfusion (I/R) did not increase lung albumin leak. However, after treatment with NG-nitro-L-arginine methyl ester (L-NAME), intestinal I/R led to increased lung leak, suggesting a protective effect of endogenous NO. The site of action of NO appeared to be the lung and not the gut because 1) after treatment with L-NAME, local delivery of NO to the lung by inhalation abolished the increase in intestinal I/R-induced lung leak; 2) L-NAME had no effect on epithelial permeability (51Cr-labeled EDTA clearance) of reperfused small bowel; and 3) after treatment with L-NAME, local delivery of NO to the gut by luminal perfusion did not improve epithelial permeability of reperfused intestines. Furthermore, L-NAME increased, and inhaled NO decreased, the density of lung neutrophils in rats subjected to intestinal I/R, and treatment with the selectin antagonist fucoidan abolished L-NAME-induced lung leak in rats subjected to intestinal I/R. We conclude that endogenous lung NO limits secondary lung injury after intestinal I/R by decreasing pulmonary neutrophil retention.

Role of capsaicin-sensitive neurons in the control of intestinal blood flow and oxygen uptake.

The effects of periarterial placement of capsaicin upon intestinal blood flow, oxygen consumption and distribution of blood flow to the vascular compartments of the gut wall were measured in anesthetized dogs. Total blood flow to the segment of distal ileum was measured with an ultrasonic blood flowmeter and arteriovenous oxygen difference (AVO2) across the intestinal segment was determined spectrophotometrically. Intestinal oxygen uptake was calculated as the product of AVO2 and intestinal blood flow. Intestinal mucosal blood flow was also measured by laser-Doppler flowmeter. Changes in blood flow distribution were estimated from the distribution of radiolabeled microspheres. Capsaicin applied periarterially induced early increase in intestinal blood flow, oxygen uptake and increase in mucosal blood flow which was higher then increase in total blood flow. The acute capsaicin-induced vascular dilation was followed by steady state response characterized by significant decrease in intestinal blood flow, oxygen uptake and redistribution of blood flow away from mucosal-submucosal the intestinal compartment. The results of these studies indicate that sensory C-fibers at rest, and when activated play a role in the control of intestinal blood flow, its distribution among vascular compartments of the intestinal wall and intestinal tissue oxygenation.

Nitric oxide mediates intestinal hyperaemic responses to intraluminal bile-oleate.

It has long been recognized that intestinal blood flow increases at mealtimes. Mesenteric hyperaemia is also evoked by activation of sensory peptidergic nerves. Our studies explored the possible role of endogenous nitric oxide (NO) in the rat intestinal vasodilator response to luminal instillation of an oleic acid plus bile mixture before and after acute intrajejunal instillation of capsaicin and after chronic pretreatment with capsaicin. In anaesthetized rats we measured jejunal blood flow (BF) with an ultrasonic Doppler flowmeter and systemic arterial pressure (AP) with a pressure transducer. Intestinal perfusion with 80 mM oleic acid in bile increased BF by 98 +/- 12%. Instillation of 4 mg of capsaicin into the jejunal lumen initially increased BF by 42 +/- 9% but was followed by vasoconstriction. Inhibition of NO synthase with 25 mg/kg i.v. N-nitro-L-arginine (L-NNA) decreased BF by 27 +/- 5% and increased AP by 37 +/- 11%. After treatment with L-NNA and after acute and chronic administration of capsaicin, the bile-oleate-induced maximal increases in BF above control levels were 42 +/- 7%, 65 +/- 12%, and 58 +/- 8%, respectively. The observed inhibitory effect of L-NNA on the intestinal hyperaemic response to the bile-oleate mixture was reversed by pretreatment with L-arginine (100 mg/kg i.v.). In capsaicin pretreated rats the subsequent bile-oleate-induced hyperaemia was reduced in magnitude but the inhibitory effects of L-NNA were proportionately the same as in animals not receiving capsaicin. These findings support the hypothesis that NO is involved with bile-oleate-induced mesenteric hyperaemia.

Intestinal ischemic disorders.

The physiology of the mesenteric circulation is described emphasizing important aspects of microcirculatory function and the factors which regulate blood flow to the bowel. Next, the pathophysiology of intestinal ischemia is considered with special focus on the disturbed mechanisms involved in ischemic disorders, such as active oxidant formation and inhibition of intrinsic protective systems. The histopathology of small intestinal and colonic ischemia and infarction is described. Finally, clinical issues are addressed including the diagnostic challenge and the management of these life-threatening disorders.

Adrenergic, purinergic, and endothelial mediators and modulators of norepinephrine-induced mesenteric autoregulatory escape.

We evaluated the effects of potential factors in autoregulatory escape from norepinephrine-induced vasoconstriction in rat anterior mesenteric artery. We determined mesenteric artery blood flow velocity with a pulsed Doppler, sonic flowmeter, and systemic arterial blood pressure with a transducer. A 4-min norepinephrine infusion (0.125-1.0 x 10(-8) M/min) intravenously evoked a dose-dependent, initial vasoconstriction that was followed by rapid escape of blood flow toward or above the control value during sustained norepinephrine administration. Neonatal capsaicin treatment enhanced vasoconstrictor responses to norepinephrine but failed to affect escape parameters. Propranolol decreased norepinephrine-induced escape dose dependently. Adenosine deaminase attenuated escape, and the combination of this enzyme plus propranolol nearly abolished escape from norepinephrine-induced vasoconstriction. Methylene blue also diminished autoregulatory escape. These findings suggest that norepinephrine-induced autoregulatory escape involves simultaneous beta-adrenoceptor, purinergic, and endothelial mediation. Norepinephrine-evoked mesenteric vasoconstriction appears to involve predominantly alpha 2-adrenoceptors and is modulated by peptidergic sensory nerves and adenosine.

Multifactorial mediation of post norepinephrine induced intestinal hyperemia.

We evaluated several potential endogenous mediators of post norepinephrine induced hyperemia in the mesenteric circulation. Hyperemia was elicited in the anesthetized rat anterior mesenteric artery after cessation of intravenous norepinephrine infusion at 0.125 to 1.0 x 10(-8) M/min x 4 min. Arterial blood pressure was measured with a transducer, and the velocity of arterial blood flow was determined with pulsed Doppler velocimetry. Conductance at the height of mesenteric hyperemia, the post norepinephrine hyperemia volume, and the autoregulatory escape volume were calculated from recorded measurements. The higher doses of norepinephrine increased both the hyperemia volume and peak conductance in both control and capsaicin pretreated rats. Hyperemic parameters were significantly diminished by pretreatment with either yohimbine, propranolol, adenosine deaminase, or methylene blue. Combining adenosine deaminase with propranolol further reduced peak conductance and the hyperemia volume compared with enzyme pretreatment alone. The magnitude of hyperemia was related to the escape volume but not to the extent of norepinephrine induced vasoconstriction. We conclude that post norepinephrine induced hyperemia in the rat mesenteric circulation is modulated by alpha 2 and beta 2 adrenergic receptor activation, adenosine release, and endothelial factors.

Regulatory issues in dentistry: how dentists can participate.

The FDA's twofold purpose of surveillance and facilitation is defined. For a successful dental program, the FDA welcomes practitioner input in reporting problems in use or performance of dental devices.

Is bradykinin (BK) a physiological vasodilator in the gut?

The physiological role of bradykinin (BK) as a mesenteric vasoregulator was explored. This nonapeptide is a potent vasodilator substance when administered exogenously in multiple in vivo models and is a smooth muscle relaxant when added to in vitro preparations. BK is naturally occurring in the gut wall. The substrate for BK, as well as the biosynthetic and metabolizing systems are present in the blood, the vascular wall, immunological cells, and perivascular neurons. BK B2 and B1 receptors have been characterized with sympathetic agonist and antagonist substances, and the receptors are present on mesenteric endothelial cells and myocytes. BK interacts with multiple endogenous mesenteric vasodilator mediators, such as nitric oxide, prostacyclin, and neuropeptides. Taken together this evidence supports the functional importance of BK as a normal vasodilator in the gut.

Adenosine modulates reactive hyperemia in rat gut.

Intestinal reactive hyperemia is an abrupt blood flow increase following release from anterior mesenteric arterial occlusion. We investigated the role of adenosine in reactive hyperemia. In anesthetized rats, mesenteric arterial velocity of blood flow was determined with pulsed Doppler velocimetry and arterial pressure with a transducer. Three indices quantifying reactive hyperemias obtained following 30, 60, and 120 s arterial occlusions included duration, the volume of blood flow exceeding preocclusion blood flow, and the percentage increase in conductance. In six rat groups (half fasted and half with intrajejunal bile-oleate solutions), hyperemia parameters were determined before and after administration of either adenosine deaminase (ADA) or two adenosine receptor antagonists, namely 8-phenyltheophylline (8-PT) and 1,3-dipropyl-7-methylxanthine (DPMX). In fasted gut the three agents had variable effectiveness against reactive hyperemia, although 8-PT was the most consistent inhibitor. Instillation of intrajejunal lipid evoked a stable hyperemia and increased duration and blood flow volume after each occlusive period. ADA and 8-PT were more effective against reactive hyperemia in fed gut than in fasted gut. Our findings suggest that adenosine is a vasodilator metabolite modulating mesenteric reactive hyperemia, especially during enhanced intestinal metabolic activity.

Bradykinin-induced mesenteric vasodilation is mediated by B2-subtype receptors and nitric oxide.

We investigated mechanisms mediating bradykinin (BK)-induced anterior mesenteric artery (AMA) vasodilation in anesthetized rats. The velocity of blood flowing (VBF) in the AMA was measured with pulsed Doppler velocimetry, and arterial pressure (BP) was measured with a pressure transducer. Drugs were infused through an intra-aortic catheter positioned proximal to the AMA origin. AMA conductance (C) was calculated from mean VBF/BP and expressed as percent of control C. BK infusion (10-1,000 ng.kg-1.min-1) increased C significantly (Cmax = 201 +/- 18%, ED50 = 100 ng.kg-1.min-1, P < 0.01 for all doses). A B2-subtype receptor antagonist, D-Arg,[Hyp3,Thi5.8,D-Phe7]BK, administered at 10(5) ng.kg-1.min-1 before or during BK infusion, inhibited the vasodilation by 73 +/- 7 and 103 +/- 7%, respectively. A nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine, administered at 5.0 mg/kg 15 min before BK, inhibited the hyperemia by 61 +/- 8%. Neither a B1-receptor antagonist nor intrajejunal capsaicin inhibited BK-induced vasodilation. BK-evoked, dose-dependent, mesenteric vasodilation in rats appears to be mediated partly by B2-receptors and endogenous NO generation.

Peptidergic nerves mediate post-nerve stimulation hyperemia in rat gut.

Cessation of perivascular nerve stimulation (NS) elicits a transient increase in intestinal blood flow above the prestimulatory value. This enhancement of blood flow constitutes the phenomenon of post-nerve stimulation hyperemia (PSH). We investigated the involvement of peptidergic sensory nerves in intestinal PSH. In anesthetized rats the velocity of blood flowing through the anterior mesenteric artery (VBF) was measured with a pulsed Doppler velocimeter. PSH was induced by 4 min of postganglionic electrical NS (5 Hz). PSH was abolished by distal periarterial application of tetrodotoxin and intra-arterial lidocaine, which suggests a peripheral sensory nervous mechanism for PSH. The increase in conductance at peak PSH was blocked by pretreatment with the selective, primary afferent neurotoxin capsaicin administered as 1) subcutaneous injection in neonatal life, 2) topical application to periarterial nerves, or 3) injection into the jejunal lumen. In rats pretreated with reserpine, NS evoked a hyperemic response, which was blocked by capsaicin. Treatment with adenosine deaminase inhibited PSH considerably less than capsaicin, suggesting a lesser role for adenosine in PSH. Our findings support the hypothesis that postganglionic NS activates both adrenergic and peptidergic nerves and that the latter release vasodilator peptides in the gut during PSH.